In the known art in this field, various devices utilizing microwave, ultrasonic wave, etc. for detecting the distance between vehicles are known. Those devices have a problem in that when two vehicles which have a device of the same kind mounted on them, the devices develop interference between them so that their use is limited to the detection of a very short distance between vehicles.
In Japanese Patent Laid Open No. 1978l6-1984, a device is disclosed which has in memory, data representing characteristic features of various vehicles, and two TV cameras for two sets of image signals in front of the vehicle, processes the two sets of image signals by feature parameters, detects the vehicle in front of the vehicle by comparing the processed data with the feature data in the memory, and then detects the distance between two vehicles by conducting a triangular survey from the spatial discrepancies in the processed data of the detected vehicle in front.
With this device there is no interference between two similar detectors when they come close together because they do not use microwave, ultrasonic wave, etc.
Because this device uses feature data of vehicles for detecting the front vehicle, it is required that feature data on all of the present running vehicles be stored in the memory, and therefore every time a new vehicle is put on the market its feature data must be newly registered or updated in the device. It is, therefore, necessary for the device to possess a memory having a vast capacity, and its contents must be renewed from time to time. This is a disadvantage, and the principle on which the device is made can not be said to be practical. Furthermore, for retrieving a feature data stored in the memory, a large amount of data processing is required and the time spent for the processing is too large, which are also disadvantages of this device.